X-ray radiation imaging systems are notably used for checking luggage in airports or in secure places. Such an imaging system comprises an X-ray radiation source subjecting the luggage to an X-ray photon flux and a semi-conductor detector receiving the X-ray photons that have passed through the luggage. As a function of the absorption of the X-ray radiation by the luggage, it is possible to analyze the content of the luggage. The analysis of the content of the luggage and the evaluation of its dangerousness are steps which are more or less complex and automated depending on whether one is dealing with luggage intended for the hold or with passenger hand luggage carried into the cabin. In either case, the objective of X-ray radiation imaging systems is to detect dangerous substances such as explosives and inflammable matter. With the evolving of requirements in regard to security, it becomes necessary to be able to determine any type of substance so as, for example, to identify the presence of several substances which, combined together, can produce explosives. The detectors of contemporary X-ray radiation imaging systems generally comprise two superposed sensitive elements. A first sensitive element detects the photons of relatively high energy and a second sensitive element detects the photons of relatively low energy. These detectors, termed dual-energy, integrate the photons over the whole of the duration of exposure and lead to the measurement of two quantities of photons in two different energy ranges. They make it possible mainly to differentiate organic materials from inorganic materials. They also make it possible to determine the density of the materials traversed by the X-ray photons and therefore their composition. However, this density determination is not precise and leads to detection ambiguities. A reason for this lack of precision stems from the fact that the energy ranges of the two sensitive elements partially overlap. In practice, contemporary X-ray radiation imaging systems have difficulty in differentiating, among organic materials, certain commonly used materials from explosive materials.
More recently, X-ray radiation imaging systems have been developed so as to approximate a spectrometric measurement in terms of energy of a photon flux. These systems comprise several processing circuits each tailored to a given energy range. Certain systems comprise up to 8 processing circuits. However, these systems exhibit the drawback of being tailored only for a certain photon flux. Now, having regard to the diversity of the materials present in luggage, the photon flux may typically vary between 104 and 108 photons per square millimeter per second, i.e. a ratio of 104. If the imaging system is tailored to a relatively small photon flux, a bigger photon flux leads to a degraded counting function, the photons no longer being counted individually. This results in a degraded image. Conversely, if the imaging system is tailored to a relatively big photon flux, the measurement of the energy of each photon is less precise, whatever the photon flux received.